Loci conferring disease resistance have been identified in many plant species. Genetic analysis of many plant-pathogen interactions has demonstrated that plants contain loci that confer resistance against specific races of a pathogen containing a complementary avirulence gene. Molecular characterization of these genes should provide means for conferring disease resistance to a wide variety of crop plants.
Those plant resistance genes that have been characterized at the molecular level fall into four classes. One gene, Hm1 in corn, encodes a reductase and is effective against the fungal pathogen Cochliobolus carbonum (Johal et al. Science 258:985-987 (1992)). In tomato, the Pto gene confers resistance against Pseudomonas syringae that express the avrPto avirulence gene Martin et al. Science 262:1432 (1993)). The predicted Pto gene product resembles a serine threonine protein kinase. The tomato Cf-9 gene confers resistance to races of the fungus Cladosporium fulvum that carry the avirulence gene Avr9 (Jones et al. Science 266:789-793 (1994). Finally, the RPS2 gene of Arabidopsis thaliana confers resistance to P. syringae that express the avrRpt2 avirulence gene (Bent et al Science 265:1856-1860 (1994)).
Bacterial blight disease caused by Xanthomonas spp. infects virtually all crop plants and leads to extensive crop losses worldwide. Bacterial blight disease of rice (Oryza sativa), caused by Xanthomonas oryzae pv. oryzae (Xoo), is an important disease of this crop. Races of Xoo that induce resistant or susceptible reactions on rice cultivars with distinct resistance (Xa) genes have been identified (Mew 1987). One source of resistance (Xa21) had been identified in the wild species Oryza longistaminata (Khush et al in Proceedings of the International Workshop on Bacterial Blight of Rice. (International Rice Research Institute, 1989) and Ikeda et al. Jpn J. Breed 40 (Suppl. 1):280-281 (1990)). Xa21 is a dominant resistance locus that confers resistance to all known isolates of Xoo and is the only characterized Xa gene that carries resistance to Xoo race 6. Genetic and physical analysis of the Xa21 locus has identified a number of tightly linked markers on chromosome 11 (Ronald et al. Mol. Gen. Genet. 236:113-120 (1992)). The molecular mechanisms by which the Xa21 locus confers resistance to this pathogen were not identified, however.
Considerable effort has been directed toward cloning plant genes conferring resistance to a variety of bacterial, fungal and viral diseases. Only one pest resistance gene has been cloned in monocots. Since monocot crops feed most humans and animals in the world, the identification of disease resistance genes in these plants is particularly important. The present invention addresses these and other needs.